Modeling glass formers: Allying with experiment to study families of complex materials and new molecular pathways to equilibration

Dartmouth College, Hanover NH

Investigators

Abstract

NON-TECHNICAL SUMMARY This award supports research and educational activities that will deepen our understanding of glassy materials, expand our ability to connect their molecular nature to their behavior, and explore new paths for how their properties might change over time. Glassy materials surround us, and many of these are made of long chain molecules known as polymers. We encounter them in items ranging from toothbrushes and airplane components to electronics and separation membranes. The span in their properties, as bulk objects and in thin films, is capable of satisfying both everyday requirements and specialized demands. These materials remain fascinating to model because there are still many challenges -- and sometimes lively disagreement -- in figuring out how to link their properties to their molecular nature and processing conditions. In some cases, polymer behavior is important to understand in order to determine the best choice of polymer for an application. One aspect of this research is to look at ‘families’ of polymers, to predict how their properties change when the local molecular structure is altered in a controlled way. An example would be to gradually increase the ‘rubbery’ nature of a polymer by introducing more points at which the long chains are pinned together. To see how this works, imagine two bowls of cooked spaghetti, with one that has had a bit of oil mixed in. Pick up a few strands of the latter with a fork and the spaghetti will slide easily back into the bowl; pick up a forkful of the former and it will be all entangled, such that you can bounce it up and down and keep it on the fork. Polymer properties change as permanent entanglements are introduced and increased, and this research will lead to a better understanding of how some of those properties evolve with structure. Another complication is that glassy polymer properties can alter over time, possibly even leading to performance failure, through a process of local changes in molecular structure known as ‘aging’. For example, leave that bowl of unoiled spaghetti sitting out for a few days and the water in it will evaporate, causing the overall volume to diminish, and the strands to become brittle. For glassy materials there is very recent experimental evidence for new pathways that lead to local changes in structure as part of aging. Working with experts on the experimental side, this work will produce new pictures and models for what kind of process might be occurring on the molecular scale in these glassy systems. The broader impact of this work will evolve in multiple ways, through the PI’s educational and outreach efforts. As an engaged scientist and as the current Dean of Science, the PI has a record of expanding the participation of all students in STEM studies and careers, and these efforts will continue. The new methods that result from this work will be made broadly available and will include outreach by researchers in the group to provide practical advice on how to implement these tools, particularly for interested experimentalists. On the educational front, the PI will continue working with students in her courses to add new reference materials about polymers and glasses to the public knowledge base. Finally, the PI will further her efforts towards increasing community and collaboration within the sciences and across disciplines throughout the campus. One successful route has been through invited fellowships and talks from visitors whose range crosses the sciences, social sciences, and humanities. TECHNICAL SUMMARY A defining feature of glassy materials is that dynamic relaxation continues to occur, even in states associated with long term stability. This work will exploit successes in modeling segmental relaxation in glassy solids as a kinetic process using the Cooperative Free Volume (CFV) rate model. A key facet of CFV is that it is informed by the thermophysical nature of the material, as described using the Locally Correlated Lattice (LCL) theoretical equation of state. Both the LCL and the CFV originated in the PI's group. There are two themes for the research: (A) Linking molecular level changes to trends in thermophysical properties by studying structurally connected families of complex materials. These will include sets of copolymers, nanocomposites, and crosslinked polymers. Probing connections between the thermal expansion coefficient and local segmental interaction strength, close-packed and free volume, and the glass transition temperature will allow a prediction about whether ellipsometry could become a diagnostic of choice for a range of predictive purposes. The CFV model will be applied to predict effects on segmental dynamics from changing molecular structure, temperature and pressure. The LCL theory will lead to predictions about miscibility within these families of interest. (B) Focusing on key molecular level processes that drive the equilibration of a material's observable macroscopic properties, such as the slow change of its overall volume (physical aging), surface adsorption, etc. One of the molecular level processes is the segmental alpha-relaxation. However, an alternative process (identifiable by dielectric measurements) has come to the forefront recently, and little is known about it. This research will advance understanding of how the mechanism works, how its dynamics are connected to that system's thermodynamic properties, and how it effects changes in volume and packing in an aging sample. The research will create new links between dynamics and thermodynamics in glassy materials. Regarding broader technical impacts: The modeling and characterization advances are applicable to a wide range of soft matter.. Connections to existing material problems will be realized through the planned collaborative efforts with experimental colleagues. The PI and her group will continue their active assistance to other experimental groups so that the models and programs from this work will advance a wider range of studies. STATEMENT OF MERIT REVIEW This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.